A commercial vehicle such as a truck or a bus is generally equipped with an engine having a high displacement. Since the commercial vehicle has the engine displacement higher than that of a passenger car, inlet and outlet ports need to have a sufficient size for a smooth gas flow which is instantly input to and output from a cylinder of the engine.
For this reason, a typical engine of the commercial vehicle uses a valve lifting method based on a center pivot scheme, in which the center pivot scheme is a type of simultaneously opening or closing a pair of intake valves or a pair of exhaust valves by one rocker arm.
The center pivot scheme may simultaneously open and close two valves to instantly input and output a large amount of air to and from the cylinder, and therefore is suitable for an engine having a high displacement.
However, the typical center pivot scheme may simultaneously operate a pair of valves but limits independent valve operations, and therefore has a difficulty in controlling a valve opening amount.
The matters described as the related art have been provided only for assisting in the understanding for the background of the present disclosure and should not be considered as corresponding to the related art known to those skilled in the art.